Narrated Anas bin Malik :
offered four Rakat of Zuhr prayer with the Prophet (p.b.u.h) at Medina and two Rakat at Dhul-Hulaifa. (i.e. shortened the ‘Asr prayer).
Indicates that Prophet (SAW) used to shorten his prayers while traveling.
“When the prayers were first enjoined they were of two Rakat each. Later the prayer in a journey was kept as it was but the prayers for non-travelers were completed.” Az-Zuhri said, “I asked ‘Urwa what made Aisha pray the full prayers (in journey).” He replied, “She did the same as ‘Uthman did.”
To understand what is being referred to in the last statement, read this.
Narrated ‘Abdullah bin ‘Umar:
“I saw Allah’s Apostle delaying the Maghrib prayer till he offered it along with the ‘Isha’ prayer whenever he was in a hurry during the journey.” Salim narrated, “Ibn ‘Umar used to do the same whenever he was in a hurry during the journey.” And Salim added, “Ibn ‘Umar used to pray the Maghrib and ‘Isha’ prayers together in Al-Muzdalifa.” Salim said, “Ibn ‘Umar delayed the Maghrib prayer because at that time he heard the news of the death of his wife Safiya bint Abi ‘Ubaid. I said to him, ‘The prayer (is due).’ He said, ‘Go on.’ Again I said, ‘The prayer (is due).’ He said, ‘Go on,’ till we covered two or three miles. Then he got down, prayed and said, ‘I saw the Prophet praying in this way, whenever he was in a hurry during the journey.’ ‘Abdullah (bin ‘Umar) added, “Whenever the Prophet was in a hurry, he used to delay the Maghrib prayer and then offer three Rakat (of the Maghrib) and perform Taslim, and after waiting for a short while, Iqama used to be pronounced for the ‘Isha’ prayer when he would offer two Rakat and perform Taslim. He would never offer any optional prayer till the middle of the night (when he used to pray the Tahajjud).”
Joining/combining prayers: it means that the worshipper joins two prayers, Zuhr and ‘Asr, or Maghrib and ‘Isha’, at the time of the earlier or later of the two prayers.
The correct view is that of the majority of scholars, which is that it is permissible to join prayers if there is an excuse for doing so, because it is proven that the Prophet (peace and blessings of Allaah be upon him) did that in places other than ‘Arafah and Muzdalifah.
Joining prayers is permissible for every traveller, and for the non-traveller if it is too difficult for him to offer every prayer on time, such as one who is sick, or if there is rain, or he is busy with some work that he cannot delay in order to pray, such as a student taking an exam or a doctor who is doing surgery and so on.
[Taken from IslamQA]